A dendrimer is a dendritic macromolecule whose structure is highly regulated, and it is a nano-size molecule having a substantially spherical shape and having a great number of functionalized terminals. Since the dendrimer has an isolated space of a nanometer scale, new functions or physical properties that conventional materials do not possess have been expected, and research thereon has been made in various fields, such as nanotechnology and biochemistry. In recent years, it has been reported that dendrimers or dendrons may be useful in a very wide field, including drug delivery, gene introduction, energy-trapping optically-active molecules, catalysts, molecular mass/molecular size standard materials, sensor/nano-scale science, and others. Thus, attention has been paid thereto.
In general, a compound that well-regulated branch structures extend three-dimensionally from the center, as seen in one of the schematic structure views illustrated below, is called a dendrimer; and a compound wherein the same structures extend only in one direction (that is, a sector or fan-shaped compound), as seen in the other view, is called a dendron.

The center of a dendrimer is called a core, and that of a dendron is called a focal point. In the dendrimer, a specific chemical bond recurs between its branches. The number of the recurrences of the specific chemical bond is represented by the wording “the number of generations (or generation number).” As the generation number becomes larger, the dendrimer becomes larger, so that the shape thereof gets closer to a sphere. Recently, books on dendrimers have been published in succession (see, for example, “Topics in Current Chemistry,” vol. 228, Dendrimer V, edited by C. A. Schalley and F. Vogtle, published by Springer, 2003; and “Science and Function of Dendrimer,” edited by Kanehiko Okada, published by IPC Ltd.). This fact demonstrates the high interest in this field.
Nowadays, the method of synthesizing a dendrimer is being considerably established. There are many reports on a divergent method, wherein the synthesis of a dendrimer is advanced outward from a core; a convergent method, wherein the synthesis thereof is advanced inward from a terminal functional group; combination of the two methods, and the like (see, for example, JP-A-2002-338535 (“JP-A” means unexamined published Japanese patent application), Chemical Review, vol. 101, 3819-3867 (2001)). Thus, the methodology thereof is being established. However, it cannot be said that a problem peculiar when high-molecular-mass compounds are handled; that is, a problem that the purification of a dendrimer is very difficult, has been sufficiently solved already.
In the divergent method, there is adopted a method of forming a branch structure onto the surface of a dendrimer (or dendron) containing a core, thereby making the number of generations of the dendrimer (or dendron) large. However, when a portion where the branch structure is not completely formed remains, it is very difficult to remove this byproduct. This difficulty increasingly becomes larger as the number of generations becomes larger.
It is said that the convergent method may become a method that avoids the difficulty of purification, which is a drawback of the divergent method. Specifically, there is adopted a method of bonding, to a focal point, plural (usually two or three) dendron molecules, whose generation number is lower by one (hereinafter referred to as the starting dendron molecules), so as to form a branch structure; therefore, the molecule species that need to be removed at the time of purification in the convergent method are the focal point moiety, the starting dendron molecules, and incomplete dendron molecules wherein a branch structure is not completely formed (hereinafter referred to as incomplete dendron molecules). According to conventional methods, incomplete dendron molecules are not easily removed; therefore, in many cases, an excess amount of the starting dendron molecules is used for the focal point moiety, thereby decreasing the incomplete dendron molecules.
However, this method has the following drawbacks: As the number of the generation becomes higher, more steps are necessary, so that the starting dendron molecules, which are valuable, are used in a more excessive amount (in vain); and further, it also becomes more difficult to remove the excessive amount of the starting dendron molecules for purification, as the number of the generation becomes higher. For this reason, there has been a strong need for development of a method that enables synthesizing a dendron effectively and purifying the dendron easily.
A thioacetal structure is generally very stable against strong acidity and strong basicity, and it can be used for being converted to a carbonyl group or as an acyl anion equivalent, or alternately, it can be reduced to methylene also. Accordingly, a thioacetal structure is used for the synthesis of various compounds. Thus, this structure is very important for organic synthesis.
As described above, usefulness of thioacetal has been recognized in a wide field for a long time. However, as to the method of the synthesis thereof, many points to be improved remain from the viewpoint of rapid, highly-effective, and widely-usable reaction. In recent years, many synthesis methods thereof have been reported (see, for example, Synlett, No. 5, pp. 727-730 (2002); Synthetic Communications, vol. 32, No. 5, pp. 715-719; and Tetrahydron Letters, vol. 43, pp. 1347-1350).
Other and further features and advantages of the invention will appear more fully from the following description.